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LED driver questions

Zeebit

Zeebit

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Hey guys,

I've done some reading but I still have some things to clear up.

I will be making an LM317 based unbranded 3W White LED driver.

Considering the dropout voltage and the LED's forward voltage, how much input voltage is needed by the driver?

Can I use a red LD test load to test the driver for the white led?
 





T

Things

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The LM317 has a dropout voltage of around 1.5V. A white LED will usually be around 3Vf, meaning you'd need a minimum input to the driver of 4.5V.

No need to use a test load on linear drivers, simply short it across your DMM on the 10A setting and you'll get the current reading.
 
FireMyLaser

FireMyLaser

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Actually, a constant current LM317 drops about 3V, and in the worst case senario, a white LED will drop 4V, so a minimum of 7V is needed.
 
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Zeebit

Zeebit

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I will be using a wallwart to power the circuit. Do I need to use a voltage regulator or will the circuit be enough to load down the supply to the correct voltage?
 
FireMyLaser

FireMyLaser

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It depends totally on the wallwart. If it is DC, then it should be fine without any other circuitry.
 
Zeebit

Zeebit

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Yeah its DC. I guess >700mA should load it down enough.

Thanks Fire and Things!
 
Zeebit

Zeebit

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Has anyone used the $1 thermal adhesive found on eBay? Is it just thermal compound or does it harden?

I know that the Arctic Alumina would be the best choice but if the cheapo works then why not.

images
 
FireMyLaser

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Adhesive would imply that it will harden, but ask the seller. They often don't know what they are selling on ebay.
 
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Zeebit

Zeebit

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It goes by many names thermal paste, compound, plaster, glue, adhesive. I know that the seller will most likely not know the real deal.

I guess I'll give it a shot. Its only a dollar anyway.

Btw, do I need to use a star shaped pad (or whatever its called) before mounting the LED the heatsink?
 
FireMyLaser

FireMyLaser

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No. But if the LED gets electrical contact with it, then you need to watch out for any other electrical contact that may be touching it.
 
T

Things

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I'd be careful using a wallwart directly, I've seen a few even 10V overspec! Would be a good idea to measure them first to make sure. I still wouldn't recommend it though.
 
Zeebit

Zeebit

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The wallwart I'm using is putting out 20V when set to 12V if there is no load. I've hooked up the driver without the LED and the input voltage dropped to 12V. I'm using 2 LEDs in series.
 
Zeebit

Zeebit

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No. Things said test loads are not needed with linear drivers. I simply hooked it up to the wallwart and shorted the output to the amps terminal on my dmm.
 
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